When I'm gluing up a picture frame, I
often "beef up" the mitered corners of the frame. To do this, I like to use an
open spline. This is just a thin piece of wood thats glued into a saw
kerf in the mitered corner of the frame. As you can see in the photo, using a
contrasting wood provides an opportunity to highlight the joint. Or you can
make it nearly invisible by using the same type of wood. Either way, the spline
adds a lot of glue surface that really strengthens the joint.

JIG -
To cut the kerf for the spline, I use a shop-made jig that straddles the
rip fence on the table saw, as shown in the drawing below.

Screwing two supports to the face of the jig at a 45° angle to
the blade forms a cradle that holds the glued-up frame securely in
place. All you need to do to cut the kerf is set the frame in place and make a
pass across the saw blade. (I use a rip blade to cut a flat-bottomed kerf.)

ASSEMBLY - Once the kerf is cut, you can glue in the spline.
After gluing and clamping the joint (see detail), remove the excess material
with a handsaw and then sand the spline flush.